With the coming of an advanced information society, the developments of optical materials such as an optical fiber and an optical wave guide in the field of optical communications, and optical materials such as a liquid crystal oriented film and a protective film for a color-filter in the field of display devices have recently advanced. In the field of display devices, in particular, a plastic substrate which is light-weight and excellent in flexibility has been studied as an alternative to a glass substrate, and the development of a display which is capable of being bent and rolled has been intensively conducted. Accordingly, there is need for a higher-performance optical material which may be used for such purposes.
Aromatic polyimides are intrinsically yellowish-brown-colored due to the intramolecular conjugation and the formation of charge-transfer complexes. Consequently, as a means of reducing coloring, methods of developing transparency, for example, by introducing fluorine atom into the molecule, imparting flexibility to the main chain, introducing a bulky group as a side chain, or the like to suppress the intramolecular conjugation and the formation of charge-transfer complexes are proposed. In addition, methods of developing transparency by the use of a semi-alicyclic or wholly-alicyclic polyimide which do not form charge-transfer complexes in principle are also proposed.
Patent Literature 1 discloses that a thin-film transistor substrate is obtained by forming a thin-film transistor on a film substrate of a transparent polyimide in which the residue of the tetracarboxylic acid component is an aliphatic group by the use of a conventional film-forming process in order to obtain a thin, light-weight and break-proof active matrix display device. The polyimide concretely used herein is prepared from 1,2,4,5-cyclohexane tetracarboxylic dianhydride as the tetracarboxylic acid component and 4,4′-diaminodiphenyl ether as the diamine component.
Patent Literature 2 discloses a process for producing a colorless transparent resin film formed of a polyimide having excellent colorlessness/transparency, heat resistance and flatness, which is used for a transparent substrate for a liquid crystal display device or an organic EL display device, a thin-film transistor substrate, a flexible wiring substrate, and the like, by a solution-casting method using a particular drying step. The polyimide used herein is prepared from 1,2,4,5-cyclohexane tetracarboxylic dianhydride as the tetracarboxylic acid component and α,α′-bis(4-aminophenyl)-1,4-diisopropylbenzene and 4,4′-bis(4-aminophenoxy)biphenyl as the diamine component, and the like.
Patent Literatures 3 and 4 disclose polyimides which are soluble in organic solvents, and prepared using dicyclohexyl tetracarboxylic acid as the tetracarboxylic acid component and diaminodiphenyl ether, diaminodiphenyl methane, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]ether or m-phenylenediamine as the diamine component.
Such a semi-alicyclic polyimide, in which an alicyclic tetracarboxylic dianhydride is used as the tetracarboxylic acid component and an aromatic diamine is used as the diamine component, combines high transparency, bending resistance and high heat resistance. However, such a semi-alicyclic polyimide generally has a great coefficient of linear thermal expansion of 50 ppm/K or more, and therefore the difference in coefficient of linear thermal expansion between a semi-alicyclic polyimide and a conductive material such as a metal is great, and a trouble such as an increase in warpage may occur during the formation of a circuit board, and there has been a problem of not easily performing a process for forming a fine circuit for use in a display, or the like, in particular.
Non Patent Literature 1 discloses a polyimide prepared using (4arH,8acH)-decahydro-1t,4t:5c,8c-dimethanonaphthalene-2t,3t,6c,7c-tetracarboxylic acid, or the like as the tetracarboxylic acid component and 4,4′-diaminodiphenyl ether (DDE), 4,4′-diaminodiphenyl methane (DDM), 4,4′-diaminodiphenyl sulfone (DDS), 3,3′-diaminodiphenyl ketone (DDK), 1,3-bis(4-aminophenoxy)benzene (BAB) or 2,2-bis[4-(4-aminophenoxyl)phenyl]propane (BAPP) as the diamine component. Non Patent Literature 2 discloses a polyimide prepared using (4arH,8acH)-decahydro-1t,4t:5c,8c-dimethanonaphthalene-2c,3c,6c,7c-tetracarboxylic acid, or the like as the tetracarboxylic acid component and 4,4′-diaminodiphenyl ether (DDE) or 1,3-bis(4-aminophenoxy)benzene (BAB) as the diamine component. Non Patent Literatures 1 and 2 disclose polyimides prepared using different stereoisomers of decahydro-1,4:5,8-dimethanonaphthalene-2,3,6,7-tetracarboxylic acid, or the like, and there is no significant difference in properties between them. Additionally, no mention is made of the coefficient of linear thermal expansion in Non Patent Literatures 1 and 2.
Patent Literature 5 discloses a polyimide prepared using decahydro-1,4:5,8-dimethanonaphthalene-2,3,6,7-tetracarboxylic acid, or the like as the tetracarboxylic acid component and 2,2′-bis(trifluoromethyl)benzidine, 2,2′-dichlorobenzidine or 4,4′-oxydianiline as the diamine component. Although Patent Literature 5 discloses that the polyimide has high transparency and has a high glass-transition temperature, no mention is made of the coefficient of linear thermal expansion. Additionally, no mention is made of steric structure of decahydro-1,4:5,8-dimethanonaphthalene-2,3,6,7-tetracarboxylic acid, or the like.
Patent Literature 6 discloses a liquid crystal alignment agent comprising a polyimide prepared using decahydro-1,4:5,8-dimethanonaphthalene-2,3,6,7-tetracarboxylic acid, or the like as the tetracarboxylic acid component and 4,4′-diaminodiphenyl methane, p-phenylenediamine or 4,4′-methylene bis(cyclohexylamine) as the diamine component. However, no mention is made of the properties such as transparency and coefficient of linear thermal expansion of the obtained polyimide, and steric structure of decahydro-1,4:5,8-dimethanonaphthalene-2,3,6,7-tetracarboxylic acid, or the like.